[llvm] [ConstraintElim] Use cond from header as upper bound on IV in exit BB. (PR #94610)

[Yingwei Zheng via llvm-commits]

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@@ -1031,6 +1031,27 @@ void State::addInfoForInductions(BasicBlock &BB) {
   WorkList.push_back(FactOrCheck::getConditionFact(
       DTN, CmpInst::ICMP_SLT, PN, B,
       ConditionTy(CmpInst::ICMP_SLE, StartValue, B)));
+
+  assert(!StepOffset.isNegative() && "induction must be increasing");
+  // Try to add condition from header to the unique exit block, if there is one.
+  // When exiting either with EQ or NE, we know that the induction value must be
+  // u<= B, as a different exit may exit earlier.
+  ConditionTy Precond;
+  if (!MonotonicallyIncreasingUnsigned)
+    Precond = {CmpInst::ICMP_ULE, StartValue, B};
+  if (Pred == CmpInst::ICMP_EQ) {
+    BasicBlock *EB = cast<BranchInst>(BB.getTerminator())->getSuccessor(0);
+    if (L->getUniqueExitBlock() == EB) {
+      WorkList.emplace_back(FactOrCheck::getConditionFact(
+          DT.getNode(EB), CmpInst::ICMP_ULE, A, B, Precond));
+    }
+  }
+  if (Pred == CmpInst::ICMP_NE) {
+    BasicBlock *EB = cast<BranchInst>(BB.getTerminator())->getSuccessor(1);
+    if (L->getUniqueExitBlock() == EB)
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dtcxzyw wrote:

> I'm not sure I understand the significance of the unique exit block here. Even if the exit is non-unique, wouldn't we be still add this condition fact to the header exit?

It may cause a miscompilation if there is a path from another exit block to the header exit.


https://github.com/llvm/llvm-project/pull/94610